We demonstrate the feasibility of optical angiography on live mice using a new photoacoustic computed tomography
(PCT) scanner. The scanner uses a sparse array of discrete ultrasound detectors geometrically arranged to capture 128
simultaneous radial "projections" through a 25-mm-diameter volume of interest. Denser sets of interleaved radial
projections are acquired by rotating the sparse array continuously about its vertical axis during data acquisition. The
device has been designed specifically for imaging laboratory mice, which remain stationary during data collection.
Angiographic data are acquired at a rate of 1280 radial projections per second following a bolus injection of 2 mg/mL of
indocyanine green (ICG).
We have designed and built a prototype PCT (photoacoustic CT) scanner suitable for small animal imaging that acquires
a sparse set of 128 photoacoustic, radial "projections" uniformly distributed over the surface of a hemisphere in response
to optical absorption from a tunable, pulsed NIR (near-infrared) laser. Acquisition of a denser set of projections is
achieved by rotating the hemispherical array about its vertical axis and acquiring additional, interleaved projections.
Each detector in the array is a 3-mm diameter, piezo-composite with a center frequency of 5 MHz and 70% bandwidth.
Spatial resolution is < 300 μm and nearly isotropic, owing to the array geometry. Preliminary results acquired at half of
the allowable laser power and with no system optimizations show a low contrast sensitivity sufficient to detect a 350 nM
concentration of a NIR-absorbing organic dye embedded in 12.5 mm of soft tissue. This scanner design will allow our
group to take advantage of HYPR (HighlY constrained backPRojection) reconstruction techniques, which can
significantly improve temporal (or spectral) resolution, without sacrificing signal-to-noise or spatial resolution. We will
report how these accelerated reconstruction techniques can be implemented with this PCT scanner design. Using this
approach, we may be able to achieve 100-ms temporal resolution for dynamic studies throughout a 20-mm-diameter
Severely premature infants are often at high risk of cerebral hemorrhage or ischemic injury due to their inability to properly regulate blood flow to the brain. If blood flow is too high, the infant is at risk of cerebral hemorrhage, while too little blood flow can result in ischemic injury. The purpose of this research is to design and develop a means of non-invasively measuring cerebral blood flow (CBF) with near infrared spectroscopy (NIRS). Such a device would greatly aid the diagnosis and monitoring of afflicted infants. Previous attempts to measure CBF with NIRS have achieved limited success. In this study we acquired high signal-to-noise NIR spectrum from 600 to 980 nm with a cooled CCD spectrometer. This spectrometer enables the differential path length factor (DPF) to be estimated with accuracy using a second derivative technique described by Matcher et al. The validity of our new approach is determined via direct comparison with a previously validated computed tomography (CT) method. Three newborn piglets were studied. CBF measurements were performed at various partial arterial CO<SUB>2</SUB> tensions (PaCO<SUB>2</SUB>) using both the NIRS and CT methods. The results of the two methods correlate well with a relationship of CBF<SUB>CT</SUB> equals -4.30 + 1.05 CBF<SUB>NIRS</SUB> (r<SUP>2</SUP> equals 0.96).
Stroke is the third leading cause of death in the United States. It is caused by ischemic injury to the brain, usually resulting from emboli from atherosclerotic plaques. The carotid bifurcation in humans is prone to atherosclerotic disease and is a site where emboli may originate. Currently, carotid stenoses are evaluated by non-invasive duplex Doppler ultrasound, with preoperative verification by intra-arterial angiography. We have developed a system that uses a color Doppler ultrasound imaging system to acquire in-vivo 3-D color Doppler images of the human carotid artery, with the aim of increasing the diagnostic accuracy of ultrasound and decreasing the use of angiography for verification. A clinical TL Ultramark 9 color Doppler ultrasound system was modified by mounting the hand-held ultrasound scan head on a motor-driven translation stage. The stage allows planar ultrasound images to be acquired over 45 mm along the neck between the clavicle and the mandible. A 3- D image is acquired by digitizing, in synchrony with the cardiac cycle, successive color ultrasound video images as the scan head is stepped along the neck. A complete volume set of 64 frames, comprising some 15 megabytes of data, requires approximately 2 minutes to acquire. The volume image is reformatted and displayed on a Sun 4/360 workstation equipped with a TAAC-1 graphics accelerator. The 3-D image may be manipulated in real time to yield the best view of blood flow in the bifurcation.